Imaging devices made of a plurality of photodetectors overlying a semiconductive substrate which is optically scanned across a field of view are well known in the art. Such imagers may use infrared photodetectors such as mercury cadmium telluride photodetectors formed on the surface of a semiconductive substrate. Alternatively, the substrate may be mercury doped germanium divided into a plurality of photodetectors. The photodetectors may be arranged in a single line to form a linear focal plane which is scanned in parallel fashion across a field of view to generate an output which is multiplexed to provide a video signal analogous to a television signal. In area focal planes, the photodetectors are arranged in a plurality of columns of photodetectors, the image being scanned in parallel with the columns to provide time domain integration of the image signal in each column, as discussed in Sequin et al., Charge Transfer Devices, Academic Press, N. Y. (1975), pp. 142-200. In both linear sensors and area sensors, system performance is limited because the optical responsivities of the individual photodetectors in the focal plane are typically nonuniform. The resulting television image will therefore be nonuniform in intensity and contrast if the variations in individual photodetectors reponsivities are significant. This problem has been solved in the prior art by providing an external source of known irradiance which is modulated at a selected frequency. An automatic responsivitiy control (ARC) circuit at the output of each photodetector selects the frequency component of the photodetector output signal corresponding to the frequency of the known source and compares its amplitude with a selected reference to adjust the amplification at the photodetector output in accordance with this comparison, so that the amplified outputs from the plurality of photodetectors respond uniformly to the external source of known irradiance. If the photodetectors are infrared sensors, the modulated source will typically be an object maintained at a known temperature shielded by a beam chopper which is rotated at the selected frequency. The ARC circuit for each detector includes a variable gain amplifier at the photodetector output connected to a digital filter turned to the modulated frequency, and a differential amplifier for comparing the output of the digital filter with a reference source to produce a voltage controlling the gain of the variable gain amplifier.
This type of automatic responsivity control therefore requires additional hardware including an object maintained at a known temperature and a rotating beam chopper or other modulating device, which adds weight, volume and complexity to the system, a significant disadvantage.